1. Field of Invention
Constant impedance transmission lines are used to move electrical signals. A transmission line is terminated at the receiving end by a resistor whose resistance is equal to the characteristic impedance of the transmission line. This terminating resistance prevents reflection of the arriving signal. Imperfections in the transmission line also cause reflections. If the source does not have a resistance equal to the characteristic impedance of the transmission line, reflected signals, arriving at the source, are reflected toward the receiver. Reflected signals cause problems. To prevent these problems the resistance of the source is made equal to the characteristic impedance of the transmission line.
2. Description of Prior Art
A transmission line driver is a circuit that drives a transmission line having a characteristic impedance of Zo. Both the driver and receiver should have a characteristic resistance of R=Zo to minimize reflections on the transmission line. There are three prior art ways of designing a terminated line driver.
A voltage source of twice the output voltage is used in series with the driving resistor Ro. Half of the source voltage is lost in Ro and the power dissipated equals the output power. FIG. 1.
A second way is to use a current source with twice the output current and a source resistance to ground. Again, the power dissipated equals the output power. FIG. 2.
The third way is to use feedback to change the value of a resistance different from the characteristic impedance to Zo. U.S. Pat. No. 6,665,399 covers this technique.